This invention relates to method and apparatus for measuring the scalar magnitude of a vector field even though the angle which the field makes with the measuring equipment may be subject to variation. It relates especially to such method and apparatus wherein the field to be measured contains at least one sinusoidally varying component, and is especially advantageous where the field contains a plurality of components at different frequencies and it is desired to obtain separate measurements of each of the plurality of field components.
One important application of the present invention with respect to which it will be described hereinafter in detail is in a system like that described and claimed in copending application Ser. No. 673,006 of Earl R. Payne, entitled ALARM PACKET SYSTEM, of even date and of common assignee herewith. In such system a receiver and an alarm actuator are secreted within a packet of real or bogus bills at the teller's location in a bank for example, and an alarm actuating field is established adjacent the exit from the bank so that when the robber removes the packet through the exit the receiver responds to removal of the packet from the exit field to actuate an alarm device in the packet, typically by producing an explosion in the packet which releases a red stain and/or a tear gas or the like. As further described in that application, it is preferred to provide, adjacent to the exit, a field having components at two different frequencies, such as f.sub.1 and f.sub.2, and to include in the receiver in the packet circuitry which will detect the strengths of each of these two fields separately, compare them, and cause an appropriate action to be taken depending upon whether the f.sub.1 or the f.sub.2 field is of greater strength. In that example the two fields are sinusoidal, low-frequency magnetic induction fields. However, the invention is applicable to the measurement of any field the orthogonal components of which can be sensed and converted to corresponding electrical signals.
It will be understood that, in the application of the invention described in detail herein, the packet containing the receiver and its sensing coils may be carried through the exit field by the robber in any orientation with respect to the field, and if one utilized, for example, a single sensing coil having a single sensing axis and the robber happened to carry the packet through the exit field with its sensing axis at right angles to the exit field, the receiver would not sense the field and the alarm would not be actuated. What is desired is for the packet circuitry to sense, and produce signals indicative of, the scalar magnitude of the field regardless of the orientation of the packet with respect to the field.
It is known that the scalar magnitude of a vector is equal to the square root of the sum of the squares of its orthogonal components; it is therefore possible to align the sensitive axes of three field-sensing transducers along three mutually orthogonal directions in the field to produce electrical signals corresponding to the orthogonal components of the vector, square these individual signals, add them together, and take the square root of the sum of their squares electronically, so as to produce an electrical indication of the scalar magnitude of the field vector.
To determine the separate scalar magnitudes of two field vectors of different frequencies occurring at the same field position, it is possible to utilize two separate systems of the type just described, each having in its signal channel, prior to the squaring device, a bandpass filter which passes the particular frequency to be measured but rejects other frequencies; in such an arrangement then, it is necessary to have a complete system for each frequency to be measured. Obviously this requires a substantial duplication of parts, which is expensive, and where accurate comparison of the strengths of the fields of different frequency is necessary as in the example described above, also requires a careful matching of the components of the two systems to each other so that equal field strengths at different frequencies will produce exactly equal output indications despite small differences in the components of the two systems and despite differences in variations of the characteristics of the components with temperature and with aging, for example.
It will be appreciated that in the specific example described above and later herein, it is the variation in the strengths of the field components of different frequencies as a function of different positions within the field which is of primary interest. However, in other applications one may be more interested in measuring the field strength at a fixed location, such field strength typically then depending upon the strength of the source of the field and upon the presence and nature of any field-affecting materials or entities. Other possible uses of the system include, without limitation, small hand-held, cased, field-strength metering instruments such as will also be described in detail hereinafter.
Accordingly, it is an object of the present invention to provide new and useful method and apparatus for the measurement of the scalar magnitude of a vector field.
Another object is to provide such method and apparatus which provides such measurement regardless of the orientation of the measuring equipment with respect to the direction of the vector field.
A further object is to provide such method and apparatus which will effect separate measurements of the scalar magnitudes of a plurality of vector field components of different frequencies, present in the same space and operative upon the measuring equipment at the same time.
Still another object is to provide the latter type of method and apparatus which is both accurate and economical with respect to the components required in the system.
It is also an object of the invention to provide such method and apparatus which do not require use of carefully measured or matched components.